1. Field
The present invention generally relates to wireless communications networks. More particularly, the present invention relates to a system and method for synchronizing timing in application specific integrated circuits (ASICs) associated with wireless communication terminals.
2. Related Art
Code division multiple access (CDMA) is one of several modulation techniques for facilitating communications in which a large number of system users are present. Although other techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other modulation techniques.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeatersxe2x80x9d and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSystem and Method for Generating Signal Waveforms in a CDMA Cellular Telephone Systemxe2x80x9d, both of which are assigned to the assignee of the present invention and are incorporated by reference. The method for providing CDMA mobile communications was standardized in the United States by the Telecommunications Industry Association in TIA/EIA/IS-95-A entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, referred to herein as IS-95.
In the above patents, CDMA techniques are disclosed in which a large number of mobile station users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations. The satellite repeaters are known as gateways and the terrestrial base stations are known as cell base stations or cell-sites. The gateways provide communication links for connecting a user terminal to other user terminals or users of other communications systems, such as a public telephone switching network. By using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA techniques result in much higher spectral efficiency than can be achieved using other multiple access techniques.
In a typical CDMA communications systems, both the remote units and the base stations discriminate the simultaneously received signals from one another via modulation and demodulation of the transmitted data with high frequency pseudo-noise (PN) codes, orthogonal Walsh codes, or both. For example, in the forward link, i.e., base station to mobile station direction, IS-95 separates transmissions from the same base station by the use of different Walsh codes for each transmission, while the transmissions from different base stations are distinguished by the use of PN codes uniquely offset in phase. In the reverse link, i.e., mobile station to base station direction, different PN sequences are used to distinguish different channels.
The forward CDMA link includes a pilot channel, a synchronization (sync)-channel, several paging channels, and a larger number of traffic channels. The reverse link includes an access channel and a number of traffic channels. The pilot channel transmits a beacon signal, known as a pilot signal, and is used to alert mobile stations of the presence of a CDMA compliant base station. After a mobile station has successfully acquired the pilot signal, it can then receive and demodulate the sync-channel in order to achieve frame level synchronization and system time etc. This feature will be discussed in greater detail below. The paging channel is used by the base station to assign communication channels and to communicate with the mobile station when the mobile station has not been assigned to a traffic channel. Finally, the traffic channels, assigned to individual mobile stations, are used to carry user communications traffic such as speech and data.
To communicate properly in a CDMA system, the state of the particular codes selected must be synchronized at the base station and the mobile station. Code level synchronization is achieved when the state of the codes at the mobile station system are the same as those in the base station, less some offset to account for any processing and transmission delay. In IS-95, this code level synchronization is facilitated by the transmission of the pilot signal from each base station which is comprised of the repeated transmission of the uniquely offset PN code (pilot PN code). In addition to facilitating synchronization at the pilot PN code level, the pilot channel also provides for identification of each base station relative to the other associated base stations using the base station""s pilot channel phase offset.
After a mobile station achieves PN code level synchronization, as stated above, it can receive and demodulate the sync channel. The synch channel carries a repeating message that specifically identifies the base station, provides system level timing, and provides the absolute phase of the pilot signal.
Consistent with the principles of the present invention as embodied and broadly described herein, an exemplary circuit includes a first communications device including at least first and second type communication paths. The first communications device is adapted to (i) receive first and second timing signals in the first type communication path and (ii) transmit data on the second type communication path. The data is transmitted in association with the received first timing signal. Next, the circuit includes a processor electrically coupled to the first communications device and configured to (i) receive the second timing signal and (ii) produce a timing word from the second timing signal. Finally, the circuit includes a second communications device including at least a first type communication path. The second communications device is coupled to the processor and adapted to receive the timing word therefrom and is configurable to receive the transmitted data and derive synchronization information therefrom. The derived synchronization information is related to the first timing signal. The second communications device also performs one or more operations in accordance with the received second timing signal and the derived synchronization information.
Features and advantages of the present invention include the ability to enhance the speed with which remotely located terminals achieve synchronization in a system where synchronization is based upon a message being repeatedly transmitted from a centralized unit to the remote terminals. By enhancing the speed of synchronization, the amount of time the remote terminal is out of service can be reduced. Additionally, the unit may operate in a lower power mode, thus facilitating a reduction in overall power consumption once synchronization has been achieved. The time savings achievable by this process amount to an average, for example, of approximately 300 ms. This in turn implies the time a mobile station is out of service due to a system loss can also be 300 ms less for each synchronization cycle. The net result is an overall savings of at least eight minutes of standby mobile phone time every hour, when the mobile phone is operating in a non-preferred system. In the non-preferred system, the total mobile phone standby time is increased by about 13%. If, for example, the mobile phone is operating in a system that experiences frequent system losses, the increase in standby time could be even higher.